Prior art of possible relevance includes the following U.S. Pat. Nos. 1,389,370 issued Aug. 30, 1921 to Metzger; 2,637,946 issued May 12, 1953 to Parks; 2,645,884 issued July 21, 1953 to Kellie; 2,885,173 issued May 5, 1959 to Dobrick; 3,254,662 issued June 7, 1966 to Wagner; 2,946,186 issued July 26, 1960 to Southam; 3,425,434 issued Feb. 4, 1969 to Strickland et al; 3,478,775 issued Nov. 18, 1969 to Friedell; 3,762,436 issued Oct. 2, 1973 to Clayton; and 4,398,393 issued Aug. 16, 1983 to Ipsen.
Pressure regulating valves of various sorts have long been known as indicated by the above identified patents. They are customarily employed in reducing the pressure of fluid from a source to a lower regulated pressure to be utilized by some apparatus at a point of use. In other words, such valves provide a controlled pressure downstream of the pressure regulating valve itself.
In some instances, it is desirable to control the pressure upstream of the pressure regulating valve. An example is a heat exchange system employing an evaporator wherein the temperature at the evaporator must be closely controlled. In the usual case, the temperature of the evaporator will be determined by the temperature at which a liquid phase fluid evaporates to become a saturated vapor; and that in turn will vary dependent upon the pressure of the vapor side of the system.
Consequently, by closely controlling the pressure on the vapor side of an evaporator, the evaporator temperature can be closely controlled.
This, in turn, requires that the pressure of the vapor side of the evaporator be controlled from a point downstream of the evaporator. To some extent, this could be accomplished by bleeding off excess pressure. However, this is not a satisfactory solution in those instances where the pressure at the vapor side of the evaporator can fall to a level less than the set point at which the system bleeds off excess pressure nor is it satisfactory in many instances where closed systems are required.
The present invention is directed to overcoming one or more of the above problems.